Swinging bucket rotors for centrifuges are known. Special swinging bucket centrifuge rotors are typically mounted to conventional centrifuges. These conventional centrifuges having a driving motor, a vertical shaft ending in an upper rotor coupling with a rotor containing the swinging buckets mounted to the coupling. Because the swinging buckets pivot outward from the rotor body giving the rotor an irregular and high windage profile, such swinging bucket rotors are usually confined in an evacuated chamber or bowl during centrifugation.
Swinging bucket rotors usually include a central supporting rotor body and-a series of peripherally mounted swinging sample tubes or "buckets." These swinging buckets are mounted symmetrically around the periphery of the central supporting rotor body at a constant radius from the spin axis of the rotor. In such rotors, material for centrifugation is placed within the bucket. This bucket is mounted for pivot or "swings" on an axis which is both tangent to the circle in which the sample is rotated by the centrifuge and normal to the spin axis of the rotor. This pivot of the bucket on an axis keeps the sample tube or bucket aligned to the force of gravity at all times.
Before the centrifuge is started, the sample containing buckets pendulously hang from the spin axis of the swinging bucket rotor parallel to the central spin axis of the rotor. When the centrifuge starts, centrifugal force causes the bucket to pivot. This pivot continues until the centrifuge bucket is normal to spin axis of the rotor.
Swinging bucket rotors rest within a central supporting rotor body during centrifugation. This central supporting rotor body is typically an all metal construction --usually titanium. Such seating of the buckets into the rotor body both complicates the construction of the rotors and the swinging buckets. A discussion of both the construction of the swinging bucket, and the periphery of the central supporting rotor body is warranted.
Each swinging bucket is typically provided with a male annulus. The rotor at its lower edge is provided with a notch having a complimentary and partial female annulus for receiving the male annulus of the swinging bucket. When the rotor begins centrifugation, the bucket swings from its position parallel to the spin axis of the rotor into a seated position normal to the spin axis of the rotor. The bucket must be seated at its male annulus into the complimentary and partial female annulus of the notch of the rotor. Such a seating of the male annulus of the bucket into the female annulus of the rotor is designed to occur when the bucket is normal to the spin axis of the rotor.
As a beginning complication, the reader will realize that the male annulus of the bucket must freely swing into the partial female annulus of the notch in the central rotor body. This requires that the partial female annulus of the notch in the central rotor body surround less than all of the bucket--otherwise swinging into the central rotor body could not occur. As a consequence, the female annulus of the notch in the central rotor body usually surrounds a little more than one half of the periphery of the swinging bucket.
Mechanisms to assist such seating of the bucket at its male annulus into the female annulus of the notch of the rotor are provided. One such mechanism is set forth in Chulay U.S. Pat. No. 4,190,195 issued Feb. 26, 1980 entitled Hanger Design for a Swinging Centrifuge Rotor.
Over simplified, the mechanism for seating the swinging bucket is discretely attached to the central rotor body. This mechanism includes a bucket attachment to a pivot axis with connection between the bucket and axis provided by a spring. When centrifugation begins, the bucket pivots on the pivot axis from a position parallel to the spin axis of the rotor to a position normal to the spin axis of the rotor. In such pivotal movement, the male annulus of the swinging bucket overlies--but does not contact--the partial female annulus of the bucket receiving notch of the rotor.
As centrifugation proceeds with increasing speed of the rotor, the centrifugal force on the now normal to the spin axis bucket--transmitted through the pivot axis--stretches the spring. The bucket moves outward against the force of the spring. The male annulus of the swinging bucket seats in the partial female annulus of the notch of the central rotor body. Once the male annulus of the bucket is seated in the partial female annulus of the notch of the rotor, the spring is essentially inactive. Centrifugation continues with the rotor proceeding to full speed with the notch of the rotor and the annulus of the swinging bucket providing the sole active connection of the bucket to the rotor. When centrifugation is complete, the rotor slows, the seating process is reversed and sample tube inside of the buckets are removed and processed further.
Such swinging bucket rotors are complicated and expensive. First, the central rotor body is of complex construction. Machining of the periphery of the rotor to provide notches having the required partial female annuluses ready to receive the male annuluses of the buckets requires great care. When it is remembered that each notch constitutes a discontinuity or "stress riser" in the periphery of the rotor which can propagate destructive rotor cracking, some of the difficulty of construction and expense in the manufacture of the central rotor body can be appreciated. It is common for the fabrication of such rotor bodies to include five or six axis metal cutting and shaping machines for producing complex and relatively smooth but complicated exterior contours to the central rotor body.
Second, installation of the swinging buckets with their contained samples to such rotors is difficult. Because of the requirement that the periphery of the swinging bucket contain notches for receiving the swinging buckets, it is required that the buckets with their sample be installed under an overlying and peripheral lip in the central rotor body. Thus, the point of bucket attachment must be reached by the operator from above the central rotor body to a point of attachment below the central rotor body which is under the protruding and notched lip of the rotor. When it is remembered that such rotors are typically spun in a centrifuge having peripheral safety armor and usually equipped for both accommodating refrigeration and a vacuum, the difficulty of this attachment can be appreciated. Specifically, such swinging buckets are attached at obstructed fastening points on the central rotor body which are removed from direct view. For example, a common swinging bucket rotor occurrence is for a centrifuge operator to fail to observe that one of several buckets is not attached to the rotor.
Third, such rotors are complicated by the mechanisms required for safe dynamic seating of the male annuluses of the buckets into the partial female annuluses on the periphery of the central rotor body. These mechanisms are elaborate and expensive being designed with the purpose of having operation of the swinging bucket rotor as safe as possible. Examples of various schemes for the seating of such swinging buckets can be found in Glasso et al U.S. Pat. No. 3,393,864 issued Jul. 23, 1968, Chulay U.S. Pat. No. 4,190,195 issued Feb. 26, 1980, and Piramoon U.S. Pat. No. 4,391,597 issued Jul. 5, 1983.
Fourth, and despite all precautions which have been developed, such rotors occasionally fail. Most typically, one of the swinging buckets is not installed properly and fails to completely seat itself to the central rotor body. When such a seating failure occurs, one bucket becomes detached, and is thrown outward from the rotor periphery with a bullet like force. The remaining swinging buckets and central rotor body then become radically out of balance on the spin axis of the centrifuge. As a result, the rest of the rotor usually comes apart with resultant damage or destruction of the rotor, the centrifuge can, and at least some damage to the spindle and drive train of the centrifuge.
What follows is a complete departure from the prior art "swinging bucket rotors."